Technical Field
The present invention relates to a manufacturing method for a mold die used for forming a mold by a thermally drooping molding method, a manufacturing method for a mold by the thermally drooping molding method, and a manufacturing method for an eyeglass lens by the mold.
Related Art
A glass mold for an eyeglass lens is formed using a mold die having heat resistance produced through a mechanical grinding and polishing process, a mechanical grinding process or an electric process, such as, electric discharge machining. Specifically, a glass mold for an eyeglass lens is obtained by heating and softening a glass material while letting the glass material contact a mold die, and by transferring a surface shape of the mold die to the glass mold. The mold die is produced by using, for example, a grinding program, for each desired surface shape.
Recently, a demand for a progressive power lens having a progressive power property, such as a near-near progressive power lens, an intermediate-near progressive power lens and a distance-near progressive power lens of a one side aspherical surface type, a double side aspherical surface type or a double side combination type, and a lens of a type similar to the progressive power lens is increasing. As a mold forming method for obtaining an eyeglass lens having a complicated free-form surface of this type, a thermal drooping molding method has been proposed as described, for example, in Japanese Patent Provisional Publications Nos. HEI 6-130333A and HEI 4-275930A. According to the thermal drooping molding method, glass material made of thermosoftening material such as glass is placed on a mold die and is softened to closely contact the mold die by being heated to a temperature higher than or equal to the softening point, so that a molding surface shape of the mold die is transferred to an upper surface of the glass material. As a result, a mold having a desired surface shape is formed.
With regard to the thermal drooping molding method, a plurality of factors including a molding surface shape of a mold die, a shape of glass material, a distance between a molding surface and a lower surface of the glass material and a heating condition could affect the molding precision. Typically, in order to ease the shape control, the molding condition is determined by fixing some of the factors and setting the other of the factors to be changeable. In general, a number of glass materials and mold dies having different shapes are prepared, and a combination of a glass material and a mold die is selected so that the distance between a molding surface and the glass material has a predetermined value (generally, the distance between geometrical centers thereof is 0.1 mm-2.0 mm). Then, preliminary molding is repeatedly performed to make a correction to the shape of the mold die, and thereafter regular molding is performed.